These type SUTs were made for specific cartridges the companies made, same for many Ortofon and Denon SUTs. I definitely would not choose this SUT.
IF it has 20db of gain, that is an x factor of 10.
cartridge output x 10 less a bit of loss will be the signal strength sent to the MM Phono RIAA Stage. That stage has a minimum signal strength sensitivity (as well as maximum). That stage has RIAA eq combined with gain up to line level to move on to an amp.
x factor 10 squared is 100. typical 47k divided by 100 = 470 ohms. That is the impedance that will be shown to the 47k input.
formula for MC loading, i.e. guidance, not hard rule is:
cartridge coil impedance x 10 = target impedance to be shown to 47k input.
working backwards: ’guidance’ match: cartridge with coil impedance of 47 ohms.
I suspect JVC had a cartridge with a fairly high (still low) output strength and a high coil impedance that this SUT was designed to match.